Neuromuscular facilitation with low-frequency stimulation and effects of some drugs

Novel Distinctive Roles of Docking Proteins in Short-term Synaptic Plasticity of Frog Neuromuscular Transmission Revealed by Botulinum Neurotoxins

Short-term synaptic plasticity (SSP) is a basic mechanism for temporal processing of neural information in synaptic transmission. Facilitation, the fastest component of SSP, has been extensively investigated with regard to Ca²⁺ signaling and other relevant substances. However, systematic analyses on the slower components of SSP, originated by Magleby and Zengel, have remained stagnant for decades, as few chemicals directly modifying these slower components have been identified. In combination with refined experimental protocols designed to study the stimulation frequency-dependence of SSP and botulinum neurotoxins A and C (BoNT-A and BoNT-C), we investigated SSP of frog neuromuscular transmission to clarify the roles of synaptosomal-associated protein of 25 kDa (SNAP-25) and syntaxin, SNARE proteins exclusively participating in vesicular events including docking, priming and exocytosis. We found that BoNT-A treatment eliminated slow potentiation, and BoNT-C poisoning abolished intermediate augmentation, two components of SSP. Fast facilitation was maintained after double poisoning with BoNT-A and -C, but the postsynaptic response became biphasic. A novel depression, termed repression, emerged by double poisoning. Repression was different from depletion because it developed even at a low-frequency stimulation of 1 Hz. We conclude that SNAP-25 and syntaxin not only play roles as cooperative exocytotic machinery, but also have roles in SSP.

Stochastic modeling of facilitated neurosecretion

Two models of neurosecretion were evaluated in terms of their ability to predict the dependency of quantal content (m) on the frequency of repetitive stimulation of a lobster motoneuron. First, the hypothesis that neurosecretion is limited by a fixed number of release sites was tested by the fit of the distribution of m by uniform and nonuniform binomial statistics. The obtained release probabilities suggest that frequency facilitation can be due to activation of a group of sites with high release probabilities. However, the fit obtained using this model is not statistically significant due to a large number of fitting parameters. Second, the hypothesis that neurosecretion is limited by the rates of exchange between the releasable pool and the total store of quanta and that each stimulus enhances quantal mobilization was tested. Monte Carlo simulation was carried out in accordance with this model and reproduced the observed distribution of m with very few fitting parameters and therefore with a high level of significance (>0.1). This result demonstrates that mobilization of extra vesicles with each stimulus is a mechanism that allows a very accurate and parsimonious quantitative description of frequency facilitation.

Facilitation at the lobster neuromuscular junction: a stimulus-dependent mobilization model

Frequency facilitation is a process whereby neurosecretion increases as a function of stimulation frequency during repetitive synaptic activity. To examine the physiological basis underlying facilitation, we have estimated the frequency dependence of the synaptic parameters n (number of units capable of responding to a nerve impulse) and P (average probability of responding) at the lobster neuromuscular junction. Both n and P increase as a function of frequency, suggesting that the efficiency of quantal docking and quantal fusion is regulated by repetitive synaptic activity. In experiments in which facilitation is strong and quantal content does not saturate over the frequency range tested, the value of P saturates at low frequencies of stimulation, and increases in quantal content at higher frequencies of stimulation are due to an increase in n. Therefore the value of P does not limit facilitation. We propose that transmitter release is limited by the rates of quantal mobilization and demobilization, and that each excitatory stimulus causes additional mobilization of quanta to dock at the presynaptic release sites. In such a model the binomial parameter n will correspond to the number of quanta docked at the release sites and available for release. We have developed and solved kinetic equations that describe how the number of docked quanta changes as a function of time and of stimulation frequency. The stimulus-dependent mobilization model of facilitation predicts that the reciprocal value of the quantal content depends linearly on the reciprocal product of the stimulation frequency and the probability of release. Fits of the experimental data confirm the accuracy of this prediction, showing that the model proposed here quantitatively describes frequency facilitation. The model predicts that high rates of quantal demobilization will produce strong frequency facilitation.

Multiple overlapping processes underlying short-term synaptic enhancement

Recently there have been exciting advances in understanding the mechanisms and functional roles of a form of short-term synaptic enhancement (STE) that results from an activity-dependent accumulation of Ca2+ in the presynaptic terminal. This form of STE is composed of at least four processes: fast-decaying facilitation (FI), slow-decaying facilitation (F2), augmentation (AUG) and post-tetanic potentiation (PTP). Recent results suggest that these processes can now be distinguished mechanistically by the site of their induction within the presynaptic terminal: FI and F2 appear to be induced by a rapid, high concentration of Ca2+ at or near the site of exocytosis, whereas AUG and PTP seem to be induced by lower levels of Ca2+ with slower kinetics, possibly within the core of the terminal. STE is highly conserved across diverse species, and appears to serve as a flexible mechanism for temporal information processing in systems ranging from peripheral motor control to higher cortical integration.

Modification of frequency augmentation-potentiation by GTP gamma S in the frog neuromuscular junction

The effect of modifiers of guanine nucleotide-binding proteins (G proteins) on the frequency augmentation-potentiation of transmitter release were studied in the frog neuromuscular junction. Using Genetransfer as a carrier the mean quantal content of the endplate potential increased by penetration of GTP gamma S into the presynaptic nerve terminal. Neither GTP gamma S alone nor carrier alone had any effect. The relationship of log (mean quantal content) versus stimulation frequency changed from a single linear to a dual linear function, suggesting that the immediately releasable pool was modified. GDP beta S + carrier also had similar effects, but was less potent. Aluminium fluoride was without effect. Extracellularly recorded presynaptic nerve action potentials remained unchanged with GTP gamma S + carrier. Also, GTP gamma S + carrier did not affect the action potential nor the cytosolic Ca2+ concentration in differentiated NG108-15 hybrid cells. It is suggested that some smg-type G protein-dependent processes are involved in determining frequency augmentation-potentiation.

Effects of vesicular acetylcholine uptake blockers on frequency augmentation-potentiation in frog neuromuscular transmission

Vesamicol inhibits the vesicular loading of acetylcholine molecules. The effects of vesamicol and similarly acting compounds on neuromuscular transmission in frogs were investigated to determine whether these inhibitors-inhibit the frequency augmentation-potentiation of transmitter release. Various vesicular acetylcholine transport blockers suppressed the stimulation frequency-related release parameter, k, in a dose-dependent manner. Artane, cetiedil, chloroquine, ethodin, quinacrine, vesamicol and its benzyl-analogue, 2-(4-benzylpiperidino)cyclohexanol, had strong effects, while those of aminacrine, chlorpromazine, fluphenazine, imipramine, pyrilamine and thioridazine were weak. A significant correlation was observed between the biochemically reported values of IC50 and the electrophysiological inhibitory potencies on k at 20 microM. Contrary to expectations from the biochemical data, however, vesamicol and its benzyl-analogue showed equipotent inhibitory actions on the electrophysiological frequency augmentation-potentiation relation. Low sensitivity and low selectivity of the frequency augmentation-potentiation for vesamicol and its benzyl-analogue lead us to conclude that the vesicular acetylcholine transporter is not the site of the electrophysiological action of vesamicol and similarly acting chemicals.

Both augmentation and potentiation occur independently of internal Ca2+ at the frog neuromuscular junction

Augmentation and potentiation of surface recorded endplate potentials (EPPs) were examined during and after tetanic nerve stimulation in both the normal and BAPTA (a Ca2+-chelator)-loaded frog neuromuscular junction (NMJ). In the BAPTA-loaded NMJ, in contrast to a great reduction of facilitation, the amplitudes and the time constants of augmentation and potentiation were almost the same as those in the normal NMJ. The slowly increasing process of transmitter release during tetanus was a little larger in the BAPTA-loaded NMJ than in the normal NMJ. These observations strongly suggest that both augmentation and potentiation occur independently of internal Ca2+ concentration.

The role of calcium in short-term potentiation in the rat hippocampal slice

Short-term potentiation of extracellular field excitatory postsynaptic potentials (EPSP's) was investigated in the stratum radiatum of CA1 of the rat hippocampal slice in vitro. A presynaptic tetanus consisting of a single train of stimuli at 250 Hz evoked short-term potentiation (STP), but not long-term potentiation (LTP) of small amplitude, just suprathreshold, EPSP's in a medium containing 2 mM Ca and 2 mM Mg. Increasing the number of stimuli in the tetanus enhanced the initial amplitude of the STP to a maximum of 40-70% with 30-40 stimuli, and enhanced the decay time constant of STP to a maximum of 60-80 s. Increasing the Ca influx by raising external Ca, lowering Mg or adding the agent 4-aminopyridine caused a large increase in the initial amplitude of STP but only a very slight increase in the decay time of STP. LTP was frequently evoked in high Ca/Mg ratios of 2.0 or 4.0, but not in 1.0 or 0.5.

Effects of 2-(4-phenylpiperidino)cyclohexanol (AH5183) and barium ions on frog neuromuscular transmission

1. By applying electrophysiological techniques such as frequency facilitation, tetanic run-down and depression, recovery from depression and post-tetanic potentiation (PTP) of the end-plate potential (EPP), the effects on frog neuromuscular transmission of 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to inhibit specifically the loading of newly synthesized acetylcholine (ACh) molecules into synaptic vesicles, and Ba2+, a selective activator of the augmentation phase of PTP, were investigated to elucidate whether these were related to ACh turnover. 2. Effects of AH5183 and Ba2+ ions were frequency dependent. At low frequency of stimulation, both agents showed essentially no effects on the EPPs recorded from Mg2+-blocked preparations. 3. AH5183 pivoted the log-linear frequency facilitation relation clockwise without altering the intercept on the ordinate, whereas Ba2+ ions did so counter-clockwise. As is the case with Ca2+ ions, Sr2+ ions shifted the relation upwards leaving its slope unaffected. 4. AH5183 selectively depressed the component of potentiation in PTP while the effect of Ba2+ ions was a specific increase in the augmentation phase of PTP. 5. Ba2+ ions increased the amplitude of EPPs in the late depressed phase during the tetanic run-down and depression experiment, but 4-aminopyridine and Ca2+ ions failed to do so. 6. AH5183 increased, Ba2+ ions reduced but Ca2+ ions did not change the constant of recovery from depression of the EPP measured on curarized preparations. 7. The present results suggested that mobilization of the ACh quanta readily available for release might be a common mechanism underlying both frequency facilitation and two components of PTP (augmentation and potentiation). The term 'frequency facilitation' would be more comprehensive if it were re-termed 'frequency augmentation' or 'frequency potentiation'.

Effects of emetine and dehydroemetine at the frog neuromuscular junction

Emetine and dehydroemetine caused concentration-dependent reduction of the quantal content of the endplate potential (EPP) at the frog neuromuscular junction. At lower concentrations, the drugs had presynaptic action only, as they decreased the amplitude of the EPP without significantly affecting that of the spontaneous miniature EPP (MEPP). At higher concentrations, the drugs had postsynaptic effects as well. Studies of low frequency facilitation indicated that although at low concentrations the effect of emetine on quantal content was independent of frequency of stimulation, at higher concentrations the effect became markedly frequency-dependent and many of the stimuli (at 4 and 8 Hz) failed to evoke EPPs. In the voltage-clamped transected cutaneous pectoris muscle emetine (10(-4) M) depressed the peak amplitude of the endplate current (EPC). Emetine markedly shortened the time constant of decay of the EPC (tau) at more negative holding potentials but did not change the single exponential character of the decay. The results indicate that the acute effects of emetine involve blocking neuromuscular transmission by acting prejunctionally at low concentrations and pre- as well as postjunctionally at higher concentrations. The prejunctional effect is due to interference with evoked release and probably, mobilization of acetylcholine. The postjunctional effect is due to blockade of the acetylcholine-activated ionic channel.

The exponent of the calcium power function is reduced during steady-state facilitation in neuron R15 of Aplysia

Stimulation of the right pleurovisceral connective at frequencies greater than 0.2 Hz produces a steady-state facilitation of the Input 1 excitatory postsynaptic potential (EPSP) in neuron R15 of Aplysia. The magnitude of facilitation is reduced by increases in the concentration of extracellular calcium. Unfacilitated EPSPs (EPSP1) exhibited a power function dependence on the concentration of extracellular calcium with an exponent (XEPSP1) of approximately 3. The exponent for facilitated release (XEPSPss) was examined by comparing steady-state EPSP amplitudes obtained in different static concentrations of calcium (Method A), or by changing the calcium concentration during continuous stimulation and determining changes in steady state EPSP amplitudes (Method B). Method A failed to reveal any dependence of XEPSPss on stimulation frequency, while Method B revealed frequency-dependent changes in XEPSPss consistent with the hypothesis that presynaptic calcium accumulation is at least partly responsible for steady-state facilitation. At a stimulation frequency of 0.5 Hz, the XEPSPss value was significantly reduced to 2/3 of the XEPSP1 value.

Pre- and postsynaptic actions of valproic acid at the frog neuromuscular junction

The effect of valproic acid (VPA) on synaptic transmission was investigated at the frog neuromuscular junction using electrophysiological methods. The drug caused a dose-dependent decrease in the quantal content of the endplate potential (EPP) and a significant reduction in the input resistance of the postjunctional membrane. There were no apparent changes in the resting membrane potential or the frequency of the miniature endplate potential (MEPP). These results suggest that VPA has pre- as well as postsynaptic actions, both of which could lead to suppression of synaptic transmission.

Dual type of facilitation at the neuromuscular junction of the frog

End-plate potentials were recorded intracellularly at the frog neuromuscular junction. Facilitation produced by conditioning shocks applied to the motor nerve was studied in two different ways: using either low-frequency trains in Mg-blocked preparations or using brief high-frequency trains followed by a test shock of variable latency in D-tubocurarine-blocked preparations. In both cases, two distinctly different types of motor end-plates were observed. The first type showed a low quantum content and a marked facilitation without depression, while the second type showed a high quantum content, a poor facilitation, and a marked depression. The physiological significance of these findings is discussed.

Adenosine effects upon transmitter release parameters in the Mg2+-paralyzed neuro-muscular junction of frog

The influence of adenosine upon the process of transmitter release was investigated in Mg2+-paralyzed frog neuromuscular junction by using conventional microelectrophysiological techniques and binomial statistical analysis. The statistical parameters used were: m (mean number of quanta released per impulse), p (the probability of quantal release) and n (store of quanta available for release). Adenosine decreased the mean quantal content (m) by reducing n. This effect appeared to be dependent upon the free intracellular Ca2+. The nucleoside competed with the transmitter releasing effects of ouabain and increased the amount of effective Ca2+ necessary for quantal release. It did not change the slope of release parameters modification during low-frequency facilitation. The data are discussed in terms of a physical model of release.

Analysis of the action of 4-aminopyridine during repetitive stimulation at the neuromuscular junction

4-Aminopyridine (4-AP) increased the quantal content (m) of end-plate potentials (e.p.p.s) evoked by continual stimulation (0.2--25 Hz) in frog end-plates depressed by Mg2+. The increase in m was due to an increase in the binomial parameter n. This was interpreted to mean that 4-AP increased the number of activated release sites. In junctions blocked by d-tubocurarine, 4-AP first increased and then decreased the amplitude of e.p.p.s. elicited during a train of stimuli of increasing frequency, indicating that 4-AP increased transmitter release more than mobilization.

Frequency facilitation and post-tetanic potentiation of a unitary synaptic potential in Aplysia californica are limited by different processes

Post-tetanic potentiation (PTP) of the monosynaptic and unitary excitatory postsynaptic potential (EPSP) recorded in cell R15 of the abdominal ganglion of Aplysia californica was observed after repetitive stimulation of the right visceropleural connective. PTP at this synapse developed after a few pulses (about 20) and after trains of low frequency stimulation (1/2 sec) under normal physiological conditions of media and temperature. No phase of post-tetanic depression was observed. Evidence is presented that the PTP is due to an increase in transmitter release. The amplitude of the PTP was a function of the frequency and number of stimuli in the preceding train. The PTP was observed to decay, with a single exponential time course, to the size of an isolated EPSP. The rate constant of PTP decay depended upon both the frequency and number of stimuli in the preceding train. The magnitude of the various types of synaptic plasticities seen at this junction, i.e., synaptic depression, frequency facilitation and PTP, correlated with the size of an isolated EPSP as well as with each other. Based on the analysis of the data in terms of a flow model of transmitter release, it is concluded that: (a) during a train of repetitive stimulation the net rate of transmitter supply into the immediately available pool (net transmitter mobilization) increases, the efficiency of the release mechanism (fractional release) increases, and the pool of immediately available transmitter depletes; (b) upon the cessation of the train, as the peak amplitude of PTP is approached, the increased but diminishing rate of net transmitter mobilization refills the available pool to its equilibrium size, while the fractional release is still elevated; (c) during the PTP period after the peak potentiation, the elevated fractional release slowly decays with a single exponential time course; (d) the size of the facilitated EPSPs during the train is limited by the net rate of transmitter supply, although the efficiency of release is also increased; while the size of the EPSPs during the falling phase of the PTP period is determined solely by an increased efficiency of the release mechanism; and (e) the rising phase of the PTP observed in the period shortly after termination of the train is produced by the refilling of the depleted pool of available transmitter in the presence of an elevated release efficiency.

Resting and stimulated values of model parameters governing transmitter release at a synapse in Aplysia californica

Transmitter release (R) at a synpase in Aplysia californica can be analyzed in terms of a model with the following parameters: A, the available pool of transmitter; F, the fraction of available pool released by a presynaptic action potential; M, the rate of transmitter mobilization into the available pool; D, the rate constant of demobilization of transmitter from the available pool. In the present paper we show that: (1) beginning with an analysis of the recovery from depression of the second of a pair of disolated EPSPs separated by a series of intervals of about 10-60 sec, and assuming that the recovery is due to refilling of a depleted A, it is possible to estimate resting equilibrium values of these parameters; (2) changes in these parameters when a new equilbrium state is reached after prolonged stimulation (e.g., 300 stimuli at 1/sec) can then be quantitatively determined; (3) the increased rate of transmitter release observed during and after repetitive stimulation is the consequence of increases in F and M with changes in A passively following; and (4) there are significant correlations among certain resting parameters and between the values of certain resting parameters and these parameters upon stimulation. Preparations with a large resting F tend to have a relatively small resting A. Preparations with a large resting F or M tend to increase these less with stimulation than preparations with smaller resting values of these parameters. Preparations with large stimulus-dependent increases in F tend to have large stimulus-dependent increases in M.

Augmentation: A process that acts to increase transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation. 2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 sec (range 4-10 sec) which is intermediate in duration between the time constants for the decay of facilitation and potentiation. 3 . The magnitude of agumentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0-3 to 0-6 following 50 impulses at 20/sec to 0-5-7-8 following 600 impulses at 20/sec. (An augmentation of 0-3 and 7-8 would increase e.p.p. amplitudes 1-3 and 8-8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased. 5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal. 6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3-8. The corresponding Q10 for the decay of potentiation was found to be about 2-4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.

Cholinergic agents affect two receptors that modulate transmitter release at a central synapse in Aplsia californica

A unitary, monosynaptic and presumably cholinergic EPSP recorded in cell R15 of the abdominal ganglion of Aplysia californica undergoes depression followed by facilitation when the presynaptic axon is repetitively stimulated at a rate of 1-3 pulses/sec. During trains of stimulation which produced this sequence of phenomena, the effects of a large number of agents known to affect cholinergic transmission in other systems were studied. The agents could be divided into 4 classes: (1) agents having no effect upon transmission at this cholinergic junction; (2) agents of a class typified by curare, which depressed all EPSPs of a train to the same extent, and which are believed to be acting in this system solely as competitive postsynaptic blockers; (3) agents typified by acetylcholine and carbachol (ACh class), which selectively depressed earlier EPSPs of a train more than later EPSPs and which appear to act by reducing the fractional release of transmitter; (4) agents typified by trimethidinium (trimethidinium class), which selectively depress later EPSPs of a train more than earlier EPSPs and which appear to act by reducing the rate of transmitter supply into the readily releasable pool. Neither the ACh class nor the trimethidinium class produced these selective effects on different pulses in the train by changes in the postsynaptic membrane potential or membrane resistance. Nor did they act by stimulating or inhibiting other recorded inputs onto R15. Iontophoretic application of acetylcholine onto R15 indicated that the effect of trimethidinium could not be explained by an alteration in desensitization of a postsynaptic acetylcholine receptor. The structural specificity of the presynaptic receptors mediating the action of the ACh and trimethidinium classes was demonstrated by the use of a larger number of structurally related compounds.

An analysis of the release of acetylcholine from preganglionic nerve terminals

1. A study has been made of the release of acetylcholine (ACh) from the preganglionic nerve terminals of the isolated superior cervical ganglion of the guinea-pig during short trains of impulses, using the amplitude of the excitatory post-synaptic potential (e.p.s.p.) as a measure of the ACh released by each impulse. 2. The time course of decay of facilitation following a single impulse could be described by two exponential components, with T1 = 200 msec, and T2 = 13-3 sec. The increase in ACh output at the beginning of stimulation at frequencies smaller than or equal to 2Hz was reasonably predicted in terms of summation of the individual facilitatory effects of each impulse in the train, but fell short of the prediction at higher frequencies. 3. The steady-state output of ACh during repetitive stimulation at frequencies between 0-5 and 20 Hz was lower than that predicted by summation of the facilitatory effects of each impulse, but reached the predicted level at frequencies smaller than or equal to 2Hz in raised Mg2+ concentrations. 4. Statistical analysis of the quantal content (m) of e.p.s.p.s evoked by each of the first five impulses in a train showed that Poisson statistics described release of ACh at the beginning of a train in most cases; when binomial statistics could be applied (two of seven axons studied), the increase in m was accompanied by an increase in the statistical parameter, n. 5. Analyses were also made of release during continuous stimulation; at the time when the steady state of release was reached, the statistical parameter, p, had also increased. Increased release of ACh at increased frequencies of stimulation was associated with increases in p in axons with p smaller than 0-5; however, in most axons (eleven of seventeen), p was greater than 0-5 in the steady state, and increases in m with frequency were due to increases in n.

On facilitation of transmitter release at the toad neuromuscular junction

1. The time dependence of the increase in amplitude (facilitation) of a second end-plate potential (e.p.p.) elicited within 10-100 msec of a preceding e.p.p. was examined at neuromuscular junctions in sartorius muscles of toads. Facilitation was defined by two characteristics, initial facilitation and the time constant of its exponential decay.2. The time constant of decay of facilitation was longer at lower temperatures and the Q(10) was 4.3 in the range 10-25 degrees C. There was no significant effect of temperature on initial facilitation.3. Ouabain (10(-4)-10(-3)M), lithium substitution for sodium, sodium azide (5 mM) and N-ethylmaleimide (NEM, 0.1 mM) initially had no effect on initial facilitation or the decay of facilitation. After some time, they all caused a longer time constant of decay of facilitation and a depression of initial facilitation.4. It was concluded that the decay of facilitation is not directly dependent on active transport of sodium ions, calcium efflux, ATP-dependent movements of calcium or mitochondrial uptake of calcium following an action potential.5. Ouabain, lithium, sodium azide, and NEM all caused an increase in transmitter release. This effect, and the late effects on facilitation, were thought to be due to an increase in intracellular calcium concentration in nerve terminals.6. No relationship was found between the quantal content of e.p.p.s (range, 0.8-100) and initial facilitation, or the time constant of decay of facilitation.7. Substitution of strontium for calcium ions caused a marked prolongation of the time constant of decay of facilitation, and a depression of initial facilitation.8. The results were consistent with the hypothesis that the time constant of decay of facilitation is related to the rate of disappearance of an ;active' complex of calcium (CaA) which, of itself, is not sufficient for transmitter release. It is suggested that an action potential produces CaA which decays with the time constant of facilitation and CaS, a short-life complex of calcium which decays with the time constant of the phasic release of transmitter. The release of transmitter is proportional to some function of [CaA] and [CaS].

The effect of tetanic and post-tetanic potentiation on facilitation of transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca.2. Estimates of f(t), the facilitation contributed by each impulse, were obtained during and following repetitive stimulation from the incremental change in e.p.p. amplitudes following step changes in the stimulation rate during the conditioning and testing trains.3. Estimates of f(t) increased during the conditioning stimulation and returned to control in the post-tetanic period. This increase in f(t) was proportional to the magnitude of tetanic or post-tetanic potentiation (PTP) present.4. These results are described by: [Formula: see text] where h(t)/h(c) is the e.p.p. amplitude at time t expressed in terms of the control, P(t) is potentiation, F(t) is facilitation and 1 is the base level of transmitter release. Thus, potentiation has a multiplicative (gain) effect on facilitation and the base level of transmitter release.5. PTP was present immediately following the conditioning train. However, if depression occurred during the conditioning train, PTP developed after a delay.6. It is suggested that facilitation and potentiation represent increases in two independent factors which act jointly to increase the probability of transmitter release.

The effect of repetitive stimulation on facilitation of transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca to characterize the processes underlying increased transmitter release during repetitive stimulation.2. There was a progressive increase in the amplitude of successive e.p.p.s during repetitive stimulation. Increasing the frequency or duration of stimulation increased this facilitation of e.p.p. amplitudes. Facilitation is defined as the fractional increase in amplitude of a test e.p.p. over a control.3. By assuming that each impulse in a train contributes an identical increment of facilitation that sums linearly with the facilitation contributed by the previous impulses, estimates of the facilitation contributed by a single impulse, f(t), were made from the incremental increase in e.p.p. amplitudes during repetitive stimulation. The average value of f(t) contributed by the first impulse in the train during stimulation at 20/sec is given by f(t) = 0.8 e(-t/50) + 0.12 e (-t/300) + 0.025 e(-t/3000),where t is in msec. The first two terms in this equation were independent of the stimulation rate used to determine f(t) while the coefficient of the third term was a function of the stimulation rate, decreasing 2 to 3 times when the stimulation rate was decreased from 20/sec to 1/sec.4. This linear facilitation model predicted growth of e.p.p. amplitudes during the first several hundred msec of repetitive stimulation. Thereafter, e.p.p. amplitudes were typically facilitated more than predicted by the linear model.5. Several new methods are presented which can be used to obtain estimates of the magnitude and time course of facilitation contributed by specific impulses during repetitive stimulation.6. It is found that the value of short-term f(t) in the tested range of 25-300 msec progressively increases during repetitive stimulation while its time course of decay remains unchanged. After 9 sec of stimulation at 20/sec, the short-term f(t) increased to 1.4 times control.7. The increase in short-term f(t) was independent of whether it was determined from a step increase or decrease in total facilitation, excluding the possibility that the observed increase in short-term f(t) resulted from a change in the rate of decay of facilitation.8. It is suggested with supporting data from the following paper (Magleby, 1973) that each impulse contributes two types of facilitation that are responsible for the growth of e.p.p.s during repetitive stimulation: a short-term facilitation with linear summation properties described by the first two terms in the expression in paragraph 3 and a long-term cumulative facilitation approximated by the third term. The long-term facilitation is expressed as an increase in both the short-term facilitation and in the base level of transmitter release. The relative contribution of these two expressions of the long-term facilitation to the third term is a function of the stimulation rate and is given by the ratio of facilitation to the base level of transmitter release.

Changes in the statistics of transmitter release during facilitation

1. The statistical nature of transmitter release during facilitation was studied at single synaptic sites by recording extracellular excitatory junctional potentials from the claw opener muscle in crayfish.2. At low temperatures, single quanta could be counted in the responses to nerve impulses. The distribution of the number of quanta observed (x) was most accurately described by assuming that x is a binomial random variable.3. A quantitative estimate was made of the effects of errors in counting quanta due to the simultaneous release of quanta and the release of quanta which were not individually detectable above the noise of the recording system. Such errors of observation cannot account for the deviation of quantal release from a Poisson distribution.4. Facilitated release occurred in the responses to the second of two closely following nerve impulses and in the responses to successive impulses in a tetanus. In both cases, the increase in the average number of quanta released (m) could be attributed entirely to an increase in the probability (p) that available quanta were released.5. The results can be interpreted most easily in terms of a model in which the maximum number of releasable quanta is limited by a finite number of discrete release sites within recording distance of the microelectrode. In this model, the binomial parameter n is an estimate of the number of these sites, and the statistical parameter p is a compound probability depending on the rate of re-occupying sites after a nerve discharge and the probability that an impulse activates an occupied site.

Cumulative and persistent effects of nerve terminal depolarization on transmitter release

1. Following focal depolarization of rat motor nerve terminals there could often be observed an ;after-discharge' of m.e.p.p.s with transient frequencies of up to 1000/sec. This after-discharge was graded with intensity and duration of the previous depolarization.2. Following pulses which were relatively short (about 1 sec) and not too large (< -100 mV local extracellular potential field) the logarithm of m.e.p.p. frequency fell exponentially. With larger or longer pulses there was a tail to the after-discharge which could persist for several minutes.3. M.e.p.p. frequency during an after-discharge was not inhibited appreciably by nerve terminal hyperpolarization, raised [Ca] (8 mM) or lowered pH.4. Measured as a multiplication of spontaneous m.e.p.p. frequency after-discharge was depressed in solution containing no Ca(2+) and added 1 mM-MgEDTA but equal in 0.125 mM-Ca(2+) or 2 mM-Sr(2+) to that in 2 mM-Ca(2+) or 8 mM-Ca(2+).5. During an after-discharge the multiplication of m.e.p.p. frequency by focal nerve terminal depolarization or raised K(+) was reduced. This phenomenon was termed ;uncoupling'.6. It was concluded that the after-discharge is not caused by a persistent rise of K(+) concentration in the synaptic cleft, nor by a maintained nerve terminal depolarization.7. In preparations depolarized by raised K(+) m.e.p.p. frequency during a relatively small focal depolarizing pulse rose continuously, after an initial rapid rise, and after the pulse there was a tail of increased m.e.p.p. frequency. The magnitude of the rise during the pulse and the tail after it were similar on, a logarithmic basis; during both processes the logarithm of m.e.p.p. frequency usually followed (approximately) an exponential time course.8. The relative magnitude of the slow effect of depolarization, as compared with the fast effect, was increased by lowering [Ca] or increasing [Mg], and the slow effect of depolarization in contrast to the fast effect was found to exist in the presence of Ca reduced to about 10(-7)M, but only during pulses. At this [Ca] there was no rapid response to depolarization. At [Ca] about 10(-10)M, there was no response at all of m.e.p.p. frequency to nerve terminal depolarization.9. The results are discussed, and compared with similar data referring to ;facilitation' and ;post-tetanic potentiation'. It is concluded that these and the slow effect of depolarization represent the same phenomenon, a response of the transmitter release system which can be distinguished from the fast response in terms of ionic requirement as well as time course.

Changes in statistical parameters during facilitation at the crayfish neuromuscular junction

1. Transmitter release at excitatory neuromuscular junctions of the crayfish was studied at different frequencies of stimulation ranging from 1/sec to 20/sec.2. Over this frequency range the average number of quanta released per stimulus (m) increased with frequency by a factor of 6-7.3. Analysis of the fluctuations in quantal release using binomial statistics indicated that the increase in m was associated with increase in the average quantal release probability (p) at stimulation frequencies between 5/sec and 20/sec. Between 1/sec and 5/sec there was an apparent increase in the number of quanta available for release (n).

The effect of temperature change upon transmitter release, facilitation and post-tetanic potentiation

1. End-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) were intracellularly recorded from rat diaphragm phrenic nerve preparations in vitro at temperatures between 7 degrees and 40 degrees C.2. The quantal content of e.p.p.s and the frequency of m.e.p.p.s showed broadly similar relationships with temperature, with maxima about 20 degrees and above 39 degrees C.3. Analysis of the change in e.p.p. quantal content showed that the maximum about 20 degrees C was accompanied by a similar maximum of p, the probability of release of quanta. The maximum above 39 degrees C was associated with a rise in n, a presynaptic store of material needed for release.4. The rate at which transmitter could be mobilized was linear in an Arrhenius plot with an apparent activation energy of 25 kcal deg(-1).5. Facilitation and post-tetanic potentiation (PTP) were shown to be entirely attributable to changes in p.6. It is suggested that facilitation and PTP have a common basis and that the (temperature-dependent) rate of Ca removal from intracellular sites at which it exerts its action is as important a determinant of the magnitude of quantal release as is the amount of Ca combining with these sites.